The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services. The three axis orientation of these spacecraft with respect to celestial bodies such as the earth and sun must be controlled in a desired attitude within a narrow tolerance.
An orbiting spacecraft is subjected to a disturbance torque environment sufficient to perturb the spacecraft from the desired attitude, at least in the absence of counteracting torques from a spacecraft attitude control system and/or propulsion system. A major component of the disturbance torque environment has a substantially periodic nature. For example, as disclosed in McGovern, et al., U.S. Pat. No. 6,672,544 (hereinafter, McGovern), assigned to the assignee of the present invention, and hereby incorporated by reference in its entirety, solar torques, i.e., torques resulting from solar radiation impingement on spacecraft surfaces, have a sinusoidal profile, and represent an important disturbance torque component for an Earth-pointing satellite.
Spacecraft attitude control systems and/or propulsion systems counteract these and other attitude disturbances using a combination of attitude sensors and actuators. For example, orbiting spacecraft often employ Earth sensors, which provide continuous measurements of roll and pitch. By responding to these measurements with an appropriate set of commanded control torques, using feedback control techniques, measured roll and pitch angles may be kept close to commanded roll and pitch angles.
Control torques may be generated by a number of known devices and methods. For example, spacecraft torque actuators may include chemical and/or electric thrusters, magnetic torquers, and momentum storage systems including one or more momentum and/or reaction wheels (hereinafter, “reaction wheel(s)”. In addition, control torques may be generated by solar sailing techniques. The latter techniques typically require the spacecraft's solar arrays to be offset from a nominal solar normal position (at which position, power generation efficiency is maximum), in order to produce a torque. For example, in a known technique, a solar panel array configuration is adjusted when a roll angle error of a spacecraft exceeds a determined threshold value, so as to produce a torque sufficient to reduce the roll angle error. Solar sailing techniques generally require a significant array offset from solar normal, resulting in a substantial power loss penalty to the spacecraft.
Momentum storage systems have been employed to store accumulated momentum resulting from the disturbance torque environment, and thereby reduce the pointing disturbance and propellant usage associated with a thruster actuation. These systems, consisting of one or more reaction wheels, have a storage capacity that may be described in terms of a permissible range of wheel speeds. As a result, a momentum management strategy must use thrusters or other actuators to unload momentum in order to prevent wheel speeds from going outside the permissible range.
As disclosed by McGovern, when the disturbance torque profile is known in advance, a prediction of wheel speed profile may be made with reasonable accuracy. The prediction can be used for a more fuel-efficient momentum management strategy. Nevertheless, according to the disclosure of McGovern, momentum unloads using thrusters are required, resulting in a significant propellant expenditure, with a consequent shortening of spacecraft maneuver life.
As a result, improved techniques of managing spacecraft momentum are needed.